Supergirl season 6 gives the Girl of Steel her most grueling mission yet, one that requires her to endure and escape the hellscape of the Phantom Zone. That challenge began in earnest in season 6 episode 2, "A Few Good Women," when the Phantoms made Supergirl relive the deaths of almost everyone close to her in the final battle with Reign in season 3. While the Phantoms may consider this their opening act to break their newest prisoner, the memory plants the seed for how Supergirl’s survival will hinge on her connection to Mon-El and the Legion of Superheroes.
Supergirl's friends recruit an alien named Silas to help rescue Supergirl. Having broken into the Phantom Zone before in a failed attempt to save his husband, Silas tells them that the Phantoms feed on fear by recreating or creating what their prisoners fear most and making them relive it. He says their torture infects what's good and uses it against the prisoner. In the Phantom Zone, Zor-El warns Kara that the more she resists and tries to escape, the worse the Phantoms will make her living nightmares.
Kara reveals to Zor-El that the Phantoms made her relive the moment where a dying Reign killed Kara’s mother Alura, Mon-El, Sam, Brainy, and Imra, in addition to severely wounding J’onn. This traumatic memory presents itself as a key to Supergirl surviving the Phantom Zone and retaining her sanity: although that was her greatest defeat, she was able to undo the horrific event by using Mon El's Legion Ring to breathe in space and turn back time to save everyone.
Hope has always been Supergirl’s greatest power. She wielded it to defeat Myriad and nullify the fear and anger stirred by the children of Liberty. With hope, she has inspired others to join her battles, both in the present and in the far future where the Legion carries on her legacy. The second episode of the final season uses this significant memory to set up a scenario where Supergirl’s connection to the Legion is the deciding factor in her withstanding the Phantoms’ torture.
Neither viewers nor Kara can trust anything that is shown in the Phantom Zone, including the man who presents himself as Zor-El. It’s no coincidence that this man, for all purposes a stranger, begins by trying to convince the Paragon of Hope to give in to hopelessness. To navigate the nightmares, Supergirl will need a physical object or memory to anchor her to reality, to prevent the outcome Silas described of being unable to distinguish between reality and nightmare. Her very first vision suggests this talisman will be the Legion Ring that was gifted to her by Mon-El in the season 3 finale after that battle with Reign. The Ring is also a beacon to summon the Legion.
Season 6 final is full of breadcrumbs leading to significant events. First, the multiple Cat Grant mentions on Supergirl likely foreshadow her reappearance on the show, even if it is only the younger Cat Grant set to be introduced in flashbacks or dream sequences. Nia dreams up a vision of Midvale High School, which will be the central location for two episodes this season. Not only has Supergirl relived that fight alongside Legion members Mon-El, Brainy, and Imra, but elsewhere in the episode, Brainy gave Dreamer a long pep talk, which was completely centered around the Legion’s connection to Supergirl. In a season where Supergirl evaluates the positive impact she has made on the world, there’s little doubt that a visit, or even a rescue, from a future generation of heroes and protectors following her creed, would be a fitting tribute to Supergirl’s legacy.